The invention is in the field of electronic reproduction technology and is directed to a method for producing and evaluating a sample cut in an electronic engraving machine for engraving printing forms, particularly printing cylinders, for rotogravure.
In an electronic engraving machine, an engraving element with an engraving stylus as a cutting tool moves in an axial direction along a rotating printing cylinder. The engraving stylus controlled by an engraving control signal cuts a sequence of depressions, called cups, arranged in an engraving raster into the generated surface of the printing cylinder. The engraving control signal is formed in an engraving amplifier by superimposition of image signal values that represent the gradations between “light” (white) and “dark” (block) to be engraved with a periodic raster signal (vibration). Whereas the raster signal effects a vibrating lifting motion of the engraving stylus for generating the engraving raster, the image signal values determine the geometrical dimensions of the cups engraved into the generated surface of the printing cylinder.
So that the cups engraved when engraving the printing form correspond to the gradations prescribed by the image signal values, the engraving control signal must be calibrated. For that purpose, sample cups for predetermined gradations are engraved before the engraving of the printing form with what is referred to as a sample cut, for example for the gradations “light” and “dark”. After the sample cut, the actual geometric dimensions of the engraved sample cups are identified and compared to the rated geometrical dimensions of those cups that represent the gradations prescribed for the sample engraving. Setting values, for example, for the parameters “light”, “dark”, and “vibration”, are acquired from the comparison of the geometric dimensions, the engraving amplifier being calibrated with these such that the cups in fact produced in the later engraving correspond to the cups required for a gradation-correct engraving.
After the sample cut, the actual geometric dimensions of the engraved sample cups, for example the transverse diagonals, the longitudinal diagonals, the widths of the penetration and the web widths must be identified. Of the actual geometrical dimensions ensues by measuring the sample cups with a
Earlier, the determination of the actual geometrical dimensions ensued by measuring the sample cups with a measuring microscope put in place on the printing cylinder and having an integrated scale.
WO-A-9 419 900 already discloses a method wherein the actual geometrical dimensions of the engraved sample cups are measured in a video image registered with a video camera.
In practice, the demand for a more exact evaluation of the video image of the sample cups is present in order to improve the engraving quality.